1. Field of the Invention
The invention relates to a pretreatment method for saccharification of a plant fiber material during saccharification of a plant fiber material that forms a monosaccharide by hydrolyzing the plant fiber material, and to a saccharification method.
2. Description of the Related Art
Biomass in the form of plant fiber has been proposed for effective use as food or fuel by decomposing, for example, sugar cane bagasse or wood chips to form sugars consisting mainly of glucose and xylose from cellulose and hemicellulose and using the resulting sugars, and this plant fiber is currently being used practically. Attention is being focused particularly on a technology for producing alcohols such as ethanol for fuel by fermenting monosaccharides obtained by decomposition of plant fiber. Various methods have been previously proposed involving the production of sugars such as glucose by decomposing cellulose and hemicellulose, an example of a typical method thereof consists of hydrolysis of cellulose using sulfuric acid, such as dilute sulfuric acid or concentrated sulfuric acid, or hydrochloric acid. In addition, other methods use cellulase enzyme, a solid catalyst such as activated charcoal or zeolite, or pressurized hot water.
However, methods that hydrolyze cellulose using an acid such as sulfuric acid present difficulty in separating the catalyst in the form of the acid and the sugar produced from the saccharification reaction mixture obtained as a result of hydrolysis. This is because glucose, which is the main component of hydrolysis products of cellulose, and acid, which serves as the catalyst of hydrolysis, are both soluble in water. Removal of acid from a saccharification reaction mixture by neutralization or ion exchange and the like not only results in increased complexity and costs, but also has difficulty in completely removing the acid, thereby frequently causing acid to remain in the ethanol fermentation process. As a result, even if the ethanol fermentation process is adjusted to the optimum pH for yeast activity, the activity of the yeast decreases due to the increased concentration of acid, thereby leading to a decrease in fermentation efficiency.
In the case of using concentrated sulfuric acid in particular, a large amount of energy is required to remove the sulfuric acid since it is extremely difficult to remove the acid to a degree that does not deactivate the yeast in the ethanol fermentation process. In contrast, in the case of using dilute sulfuric acid, although the sulfuric acid can be removed comparatively easily, energy is again required since the cellulose must be decomposed under high temperature conditions. Moreover, it is extremely difficult to separate, recover and reuse acids such as sulfuric acid or hydrochloric acid. Consequently, the use of these acids as catalysts for glucose formation is one of the causes that drives up the cost of purifying bioethanol.
In addition, in methods that use pressurized hot water, it is difficult to adjust conditions and form glucose at a stable yield. Not only is there the risk of the glucose also decomposing resulting in a decrease in glucose yield, but there is also the risk of the function of the yeast being decreased by decomposition components, thereby inhibiting fermentation. Moreover, the reaction apparatus (supercritical apparatus) is expensive while low durability also causes problems in terms of cost.
Japanese Patent Application Publication No. 2008-271787 (JP-A-2008-271787) and Japanese Patent Application No. 2008-145741 disclose that a cluster acid in a pseudo-molten state or dissolved state has superior catalytic activity with respect to decomposition of cellulose and is easily separated from sugars produced. According to this disclosed technology, differing from the concentrated sulfuric acid method and dilute sulfuric acid method described above, together with enabling recovery and reuse of the hydrolysis catalyst, energy efficiency of the process from hydrolysis of cellulose to recovery of an aqueous sugar solution and recovery of the hydrolysis catalyst can be improved.
However, naturally-occurring plant fiber materials such as wood chips or bagasse contain lignin in addition to cellulose and hemicellulose, and these components are present in the form of complex mixtures. Lignin lowers the ease of contact of cellulose and hemicellulose with catalyst; thereby impairing the saccharification reaction thereof. In addition, since wood-based plant fibers have water-repellent pectin on the surface thereof, these fibers mix poorly with the catalyst and water. Consequently, it is difficult for cluster acid or water to penetrate into wood-based plant fibers, thereby lowering the saccharification reactivity of the cellulose and hemicellulose. As has been previously described, naturally-occurring plant fiber materials, and particularly wood-based plant fiber materials, are susceptible to decreases in saccharification rate due to decreases in reactivity of the cellulose and hemicellulose attributable to lignin and pectin. Thus, in order to increase the saccharification reactivity of plant fiber materials according to the above-mentioned disclosed technology, it is necessary to carry out pretreatment in advance so as to facilitate reaction of cellulose in the presence of lignin, for example.